White teeth are considered cosmetically desirable. Unfortunately, teeth become discolored during the normal aging process and when exposed to substances such as tobacco, certain chromogens in foods and beverages, such as tea and coffee, and medicinal substances.
The components of teeth that acquire stains are enamel, dentin, and pellicle. Tooth discoloration results from both extrinsic and intrinsic staining. Extrinsic staining of the tooth surface arises as a result of the accumulation of various chromogenic substances (in addition to chromogen precursors, which are initially colorless, but later chemically convert to chromogens) within the pellicle. This type of staining may be reduced by removing the pellicle layer containing the adherent chromogens, such as with mechanical abrasion. As the pellicle layer reforms in the mouth, extrinsic staining may reoccur and require subsequent mechanical removal. Intrinsic staining occurs as a result of chromogenic substances derived from sources within the tooth, or from stains derived from external sources that penetrate into the tooth, particularly the enamel layer. Intrinsic staining is not amenable to removal by mechanical methods of tooth cleaning. This type of staining may be reduced by using chemical agents that oxidize or solubilize chromogens in the enamel layer.
Tooth-whitening compositions generally fall into two categories: (1) liquids, powders, gels, or pastes that mechanically or superfluidly abrade or erode stains located in the pellicle layer; and (2) liquids, gels, or pastes that chemically oxidize the stains in the pellicle and in the enamel or dentin components. Tooth-whitening compositions may be used by professionals in dental offices and by consumers in residential settings. The majority of professionals use mechanical abrasion to remove stains and tooth-whitening compositions that oxidate and remove dental stains. Current tooth-whitening compositions that utilize oxidizers are applied to the teeth for a period of time often greater than 30 minutes, and sometimes as long as 8 to 12 hours per day for 1 to 2 weeks in order to produce noticeable stain reduction. The slow rate of whitening is in large part the consequence of formulations that are developed to maintain stability of the oxidizing composition prior to use. These oxidizing compositions typically contain an oxidizing agent, such as hydrogen peroxide or a hydrogen peroxide precursor, e.g., carbamide peroxide, sodium peroxide, or calcium peroxide, which is mixed with an anhydrous or low-water content, hygroscopic viscous carrier containing glycerin and/or propylene glycol and/or polyethylene glycol. Some oxidizing compositions include up to about 38% by weight of hydrogen peroxide.
Prolonged exposure of teeth to current whitening compositions has a number adverse effects. These include: 1) sensitizing the tooth to heat, cold, and overly sweet substances possibly due to the use of peroxides and hygroscopic viscous carriers, 2) irritating mucous membranes with high concentrations of peroxide, 3) solubilizing calcium components from the enamel layer at pH level less than 5.5 with associated transient demineralization, 4) penetrating the intact enamel and dentin by the whitening agents, so as to reach the pulp chamber of a vital tooth thereby risking minor penetration of pulpal tissues, and 5) forming reactive species that have been implicated in the formation of dysplastic cells.
There is a need for whitening teeth while avoiding the adverse effects usually associated with prolonged exposure to common tooth whitening compositions.